1. Field of the Invention
This invention relates to an apparatus for removing Au or Ag plating films deposited on the surface of an area for which plating is not necessary, from a leadframe depositing the plating films.
2. Description of Related Art
As one of systems for mounting semiconductor chips on a leadframe, a so-called wire bonding system is known in which each of the semiconductor chips is bonded to each of the die pads of the leadframe and is connected by bonding wires with the tips of inner leads.
An example of the leadframe used in this system is shown in FIGS. 1A and 1B. In order to facilitate a bonding with the bonding wires and improve its reliability, a leadframe 1, as shown in FIG. 1A, has plating films 2 of precious metal, such as Au or Ag, deposited on the surfaces of parts (indicated by hatching), such as a die pad 1a and the tips of inner leads 1b, for which plating is necessary. If the deposition of the plating films is extended unnecessarily, a greater amount of precious metal, such as Au or Ag, than is necessary will be used, and thus cost is raised. Moreover, if the Au or Ag plating films protrude from a mold area (an area in which an IC is covered with resin), the effect of the hermetical seal of molding will be lost or molding resin will be peeled off. This causes various obstacles to a semiconductor mounting process. Consequently, all the products depositing the plating films outside areas for which plating is necessary are treated as rejects. In this way, when the plating films are deposited outside the areas for which plating is necessary, production efficiency is impaired. The positioning accuracy of the plating films is thus important in plating the leadframe.
In general, the plating films are deposited in such a way that, for example, a rubber pad is pressed against the surface of the area of the leadframe for which plating is not necessary and an Au or Ag plating liquid is sprayed on the leadframe. In this case, the plating liquid penetrates into clearance between the rubber pad and the leadframe, and films may be deposited by the plating liquid on the surface of the area for which plating is not necessary. The plating films deposited on the surface of this area are less in thickness than those deposited on the surface of an area for which plating is necessary.
Thus, as shown in FIG. 1B, when the Au or Ag plating films 2 are also deposited on the surface of an area A of the leadframe 1 for which plating is not necessary, an apparatus for removing films, such as that shown in FIG. 2, is used to eliminate unwanted plating films. Specifically, the leadframe 1 is passed between insoluble anode rollers R so as to function as anodes, and an insoluble cathode plate P is used for electrolysis through a film removing liquid L such as a cyanic solution.
This electrolytic treatment is carried out in accordance with the thicknesses of the plating films deposited on the surface of the area for which plating is not necessary, over the entire surface including the area for which plating, such as Au or Ag, is necessary, so that the plating films are dissolved and removed.
With such a technique of removing the plating films, however, the plating films in the area for which plating is necessary are removed by a predetermined thickness, and thus the thicknesses of the Au or Ag plating films themselves must be previously increased. Consequently, time required for plating is extended unnecessarily and production efficiency is impaired accordingly, with a resulting increase in cost.
Furthermore, the use of this technique shows a tendency that the plating films deposited inside the area for which plating is necessary are excessively removed or the plating films outside this area cannot be completely removed. In this way, it is very difficult and cumbersome to adjust only necessary plating films to desired thicknesses so that they are kept constant.